1. Field of the Invention
The present invention relates to a zoom lens system for camera, and in particular, to a zoom lens system for a lens-shutter-type compact camera.
2. Description of the Prior Art
Unlike a zoom lens system for a single lens reflex (SLR) camera which requires a space for a quick-return mirror behind the photographing lens system, a zoom lens system for a compact camera does not require a long back focal distance.
As a zoom lens system of this type having no specific requirement on the back focal distance, the three-lens-group arrangement, i.e., a positive lens group, a positive lens group and a negative lens group in this order from the object, have been proposed. More specifically, a zoom lens system of the three-lens-group arrangement has been disclosed in, e.g., U.S. Pat. No. 4,978,204. However, in the above zoom lens system, if the zoom ratio thereof is increased, the overall length thereof at the long focal length extremity becomes longer.
Furthermore, in order to miniaturize the zoom lens system, and to obtain a higher zoom ratio, a zoom lens system of the four-lens-group arrangement, i.e., a positive lens group, a negative lens group, a positive lens group and a negative lens group in this order from the object, has been proposed, as disclosed in, e.g., JUPP6-265788 and JUPP2000-180725. Even by such a four-lens-group arrangement, the overall length of the zoom lens system at the long focal length extremity becomes longer due to the long traveling distances of the lens groups for zooming; and miniaturization of the zoom lens system cannot be attained, since the entrance pupil is distant from the front (most object-side) lens group at the short focal length extremity, so that the diameter thereof becomes larger.
Still further, if an attempt is made to obtain both a higher zoom ratio and the shorter overall length of the zoom lens system at the long focal length extremity, the focusing sensitivity thereat increases, so that the focal point moves largely even with respect to a slight positional error caused by the movement of each lens group. Consequently, the image-forming performance deteriorates.
Particularly, in the case where a higher zoom ratio is attained by a zoom lens system of the four-lens-group arrangement, i.e., a positive lens group, a negative lens group, a positive lens group and a negative lens group in this order from the object, such a four-lens-group arrangement is advantageous to shorten the overall length of the zoom lens system at the long focal length extremity, in comparison with a zoom lens system of the three-lens-group arrangement; however, in the prior art, the focusing sensitivity of internal lens groups (the second and the third lens groups) cannot be reduced.
The present invention is to provide a miniaturized zoom lens system with a higher zoom ratio, which can achieve suitable optical performance over the entire focal length ranges, in spite of a small number of lens elements in the zoom lens system. These features can be attained by making the overall length of the zoom lens system at the long focal length extremity shorter, and by making the diameter of the front (most object-side) lens group smaller.
Furthermore, the present invention is to provide a miniaturized zoom lens system of the four-lens-group arrangement, i.e., a positive lens group, a negative lens group, a positive lens group and a negative lens group in this order from the object, which is advantageously small for manufacturing and has a higher zoom ratio, while the focusing sensitivity which tends to become larger in the internal lens groups (the second and the third lens groups) can be reduced.
Still further, the present invention is to provide a zoom lens system which (i) is suitable for a retractable lens-shutter-type compact camera, and (ii) has the smaller diameter of the front (most object-side) lens group.
More specifically, a zoom ratio of the zoom lens system can be set to Z(=fT/fW) greater than 3, and the overall length TLT thereof can be made shorter to the extent that TLT/fT less than 1.0;
wherein
TLT designates the distance, at the long focal length extremity, between the most object-side surface of the zoom lens system and the image plane;
fT designates the focal length of the entire zoom lens system at the long focal length extremity; and
fW designates the focal length of the entire zoom lens system at the short focal length extremity.
In addition to the above, a zoom ratio of the zoom lens system according to the present invention can be made higher, such as Z(=fT/fW) greater than 4, under the same condition of TLT/fT less than 1.0.
According to a first aspect of the present invention, there is provided a zoom lens system including a positive first lens group (hereinafter, the first lens group), a negative second lens group (hereinafter, the second lens group), a positive third lens group (hereinafter, the third lens group), and a negative fourth lens group (hereinafter, the fourth lens group), in this order from the object. zooming is performed by moving each of the first through fourth lens groups in the optical axis direction, and the zoom lens system satisfies the following conditions:
1.1 less than f23T/f23W less than 1.8xe2x80x83xe2x80x83(1) 
0.2 less than LD23W/fW less than 0.45xe2x80x83xe2x80x83(2) 
0.01 less than (D23Wxe2x88x92D23T)/fT less than 0.05xe2x80x83xe2x80x83(3) 
wherein
f23W designates the combined focal length of the second and third lens groups at the short focal length extremity;
f23T designates the combined focal length of the second and third lens groups at the long focal length extremity;
LD23W designates the distance between the most object-side surface of the second lens group and the most image-side surface of the third lens group at the short focal length extremity;
fW designates the focal length of the entire zoom lens system at the short focal length extremity;
D23W designates the axial air-distance between the second lens group and the third lens group at the short focal length extremity; and
D23T designates the axial air-distance between the second lens group and the third lens group at the long focal length extremity.
The zoom lens system according to the first aspect of the present invention further satisfies the following conditions:
0.1 less than |r1/fT| less than 0.25(r1  less than 0)xe2x80x83xe2x80x83(4) 
6.0 less than |fT/f2| less than 12.0 (f2 less than 0)xe2x80x83xe2x80x83(5) 
wherein
r1 designates the radius of curvature of the most object-side surface of the first lens group; and
f2 designates the focal length of the second lens group.
In the zoom lens system according to the first aspect of the present invention, the traveling distances of the first and fourth lens groups are the same upon zooming. In other words, these lens groups are integrally moved. According to this arrangement, the structure of the lens barrels can be simplified, and the size of the zoom lens system in the radial direction can be made smaller. As a result, a lens-shutter-type compact camera having a retractable zoom lens system can be miniaturized, and further, the structure of the lens barrels can also be simplified.
Focusing can be performed by integrally moving the second lens group and the third lens group in the optical axis direction. In this regard, for miniaturizing a camera, it is preferable that a lens group to perform focusing (hereinafter, focusing lens group) be small and light (not heavy), and that the traveling distance thereof be shorter. The second lens group or the third lens group, in which the optical-effective diameter is relatively smaller than other lens groups, can be a focusing lens group. However, if an attempt is made to perform focusing by the second or third lens group only, the traveling distance thereof becomes longer, so that miniaturization of the zoom lens system is difficult. On the other hand, by performing focusing with the integrally-moved second and third lens groups, miniaturization of the zoom lens system can be attained, and optical deterioration can be prevented when photography is carried out for an object at a closer distance.
In the first aspect of the present invention, the second lens group requires strong refractive power in order to attain a higher zoom ratio with a shorter traveling distance. In such a case, if a negative lens element of the second lens group is formed by a lens material of low refractive index, coma and astigmatism largely occur; and furthermore, a negative value of the Petzval sum becomes larger, so that field curvature largely occurs in the positive direction. Consequently, off-axis aberrations deteriorate. On the other hand, for the purpose of avoiding the deterioration of off-axis aberrations, it is preferable that the average refractive power N2G of the d line with respect to the negative lens element of the second lens group be greater than 1.82. If N2G exceeds the lower limit of this condition, the negative value of the Petzval sum becomes larger, and off-axis optical performance deteriorates.
According to a second aspect of the present invention, there is provided a zoom lens system including a first lens group, a second lens group, a third lens group, and a fourth lens group, in this order from the object. zooming is performed by moving each of the first through fourth lens groups in the optical axis direction, and the zoom lens system satisfies the following condition:
0.4 less than {(fT/f12T)2xe2x88x92(fT/f1)2}/{(fT/f123T)2xe2x88x92(fT/f12T)2} less than 1.6xe2x80x83xe2x80x83(6) 
wherein
fT designates the focal length of the entire zoom lens system at the long focal length extremity;
f1 designates the focal length of the first lens group;
f12T designates the combined focal length of the first lens group and the second lens group at the long focal length extremity; and
f123T designates the combined focal length of the first lens group, the second lens group and the third lens group at the long focal length extremity.
The zoom lens system according to the second aspect of the present invention further satisfies the following condition:
3.0 less than [{(fT/f12T)2xe2x88x92(fT/f1)2}+{(fT/f123T)2(fT/f12T)2}]/Z  less than 4.0xe2x80x83xe2x80x83(7) 
wherein
Z =fT/fW; and
fW designates the focal length of the entire zoom lens system at the short focal length extremity.
Note that {(fT/f12T)2xe2x88x92(fT/f1)2} in conditions (6) and (7) is defined as the focusing sensitivity (K2) of the second lens group, and (fT/f123T)2xe2x88x92(fT/f12T)2} is defined as the focusing sensitivity (K3) of the third lens group.
Similar to the first aspect of the present invention, the traveling distances of the first and fourth lens groups are the same upon zooming, and these lens groups are integrally moved. According to the second aspect of the present invention, the same advantages as those of the first aspect can be obtained; and at the same time, the focusing sensitivity can be reduced.
In the case where the first and fourth lens groups are integrally moved upon zooming, the zoom lens system preferably satisfies the following condition:
2.5 less than |(fT/f1)2+1xe2x88x92(fT/f123T)2|Z  less than 4.0xe2x80x83xe2x80x83(8) 
Note that (fT/f1)2+1xe2x88x92(fT/f123T)2 is equivalent to |K1+K4|/Z.
Similar to the first aspect of the present invention, focusing is preferably performed by integrally moving the second lens group and the third lens group in the optical axis direction. By integrally moving these lens groups, the same effects as those of the first aspect of the present invention can be obtained.
The present disclosure relates to subject matters contained in Japanese Patent Application Nos. 2001-137324 (filed on May 8, 2001) and 2001-160044 (filed on May 29, 2001) which are expressly incorporated herein in their entireties.